Modification of the sintering behaviour of mesophase powder from a petroleum residue

The effect of preparation conditions on the sintering behaviour of different binderless polyaromatic mesophase (BPAM) powders, obtained by extraction with different solvents of four petroleum semicokes of increasing carbonisation degree, is described. The analyses of the powders indicate that large differences in composition and structure are obtained when changing the semicoke. Increasing carbonisation degree of the semicoke produces a decrease in the amount of β-resins, increase in mesophase content, C/H ratio and aromaticity, but decreases the thermofusibility of BPAM. Increasing extraction strength of the solvent reduces the amount of light material, and thermofusibility of BPAM, being more evident with low carbonised semicokes. Selection of carbonisation conditions and solvent permits to adjust fusibility of the powders and to improve sintering. Graphitised compacts exhibited the best properties when they are produced from low carbonisation degree semicokes. In this case solvents of low extraction strength provide large thermofusibility to the material, and compacts may swell. When using semicokes of higher carbonisation degree there is a reduction of the sintering abilities of the powders, this being more evident as the strength of the solvent increases.     

A contribution to the study of the electrochemical oxidation of p-aminophenol on a mercury electrode

The reversibility of the oxidation process of p-aminophenol on a mercury electrode in buffered aqueous solutions is studied. Differential capacity curves are recorded and the number of electrons is determined by chronoamperometry. Kinetic parameters are determined at the foot of the wave.The results obtained are in agreement with the hypothesis that quinoneimine hydrolysis is slower than its diffusion, therefore the former takes place in a homogeneous medium and not in the interphase.     

Carbons from furan-polymers prepared in the presence of a double-chain amphiphile

Furfuryl alcohol was polymerized in the presence of a double-chain amphiphile using malonic acid or phosphoric acid as the catalyst, leading to an aggregate of spherical particles with a long period of 2.6 nm. On calcination at 1000 °C in nitrogen gas, the polymer particles were converted into lamella-patterned carbons with a long spacing of some micrometers that are composed of carbon layers 0.4 nm in spacing. The lamella-patterned carbon particles were further developed into a highly ordered structure in an appreciable portion on calcination at 2800 °C in argon gas. The present results demonstrate that the presence of a double-chain amphiphile in the polymerization process is effective for the synthesis of such a structurally modified carbon from non-graphitizable furan polymers.     

On the mechanism of fullerene formation in a carbon plasma

A conception about the mechanism of fullerene formation in carbon plasma is developed based on the influence of electron concentration on the stability of carbon clusters. Thermodynamic estimates for the efficiency of formation of spheroidal, flat clusters and linear carbon chains depending on the charge of the reacting particles are described. Strong influence of cluster charge upon the geometry and stability of flat clusters is shown. The charge variation of flat clusters can promote both their folding to curved structures and their dissociation. It is concluded that the fluctuations of electron concentration in a carbon plasma can result in the accumulation of fullerene clusters and the dissociation of flat clusters.     

The pull-out and failure of a fiber bundle in a carbon fiber reinforced carbon matrix composite

The interfacial failure is examined for a unidirectionally reinforced carbon fiber/carbon matrix composite. A novel tensile test is conducted which realizes the processes of interfacial debonding and subsequent pull-out of a fiber bundle from the surrounding composite medium. The critical stress at the onset of delamination cracking is related to the fracture energy (the critical energy release rate for mode II cracking). A force-balance equation of a fiber bundle, which is quasi-statically pulled-out of the composite socket, is formulated in terms of the inter- and intra-laminar shear strengths of the composite. This equation is successfully used to estimate the delamination crack length along the debonded fiber bundle, as a function of the stress applied to the bundle.     

Electronic and transport properties in circular graphene structures with a pentagonal disclination

We investigate the electronic and transport properties of circular graphene structures (quantum dots) that include a pentagonal defect. In our calculations, we employ a tight-binding model determining total and local density of states, transmission function and participation number. For the closed structure, we observe that the effect of the defect is concentrated mainly on energies near to zero, which is characteristic of edge states in graphene. The density of states and transmission functions for small energies show several peaks associated with the presence of quasi-bound states generated by the defect and localized edge states produced by both the circular boundaries of the finite lattice and induced by the presence of the pentagonal defect. These results have been checked by calculating the participation number, which is obtained from the eigenstates. We observe changes in the available quasi-bound states due to the defect and the creation of new peaks in the transmission function.     

Investigations of the pore formation in the lead selenide films using glacial acetic acid- and nitric acid-based electrolyte

ABSTRACT: We report a novel synthesis of porous PbSe layers on Si substrates using anodic electrochemical treatment of PbSe/CaF2/Si(111) epitaxial structures in an electrolyte solution based on glacial acetic acid and nitric acid. Electron microscopy, x-ray diffractometry, and local chemical microanalysis investigations results for the porous layers are presented. Average size of the synthesized mesopores with ~1010 cm-2 surface density was determined to be 22 nm. The observed phenomenon of the active selenium redeposition on the mesopore walls during anodic treatment is discussed.     

A New Tool to Study Parkinsonism in the Context of Aging: MPTP Intoxication in a Natural Model of Multimorbidity

The diurnal rodent Octodon degus (O. degus) is considered an attractive natural model for Alzheimer’s disease and other human age-related features. However, it has not been explored so far if the O. degus could be used as a model to study Parkinson’s disease. To test this idea, 10 adult male O. degus were divided into control group and MPTP-intoxicated animals. Motor condition and cognition were examined. Dopaminergic degeneration was studied in the ventral mesencephalon and in the striatum. Neuroinflammation was also evaluated in the ventral mesencephalon, in the striatum and in the dorsal hippocampus. MPTP animals showed significant alterations in motor activity and in visuospatial memory. Postmortem analysis revealed a significant decrease in the number of dopaminergic neurons in the ventral mesencephalon of MPTP animals, although no differences were found in their striatal terminals. We observed a significant increase in neuroinflammatory responses in the mesencephalon, in the striatum and in the hippocampus of MPTP-intoxicated animals. Additionally, changes in the subcellular expression of the calcium-binding protein S100β were found in the astrocytes in the nigrostriatal pathway. These findings prove for the first time that O. degus are sensitive to MPTP intoxication and, therefore, is a suitable model for experimental Parkinsonism in the context of aging.     

Suppressing the effect of cullet composition on the formation and properties of foamed glass

The process of foaming glass is very dependent on the chemical composition of the glass. In this study we used a foaming-agent/oxidizing-agent couple and a crystallization inhibitor to foam cullets of flat, container and CRT-panel glass. Foamed glass with a density of 110–120?kg?m^–3, a thermal conductivity of 50–52?mW?m^–1 K^–1 and a homogeneous pore structure was obtained from a mixture of panel glass, 0.33?wt% carbon and 4.45?wt% Fe₂O₃. We also showed that it is possible to fabricate foamed glass with the same density or pore structure as mentioned above by adding up to 50?wt% container cullet or 70?wt% flat glass to the mixture. In the foamed samples with a low content of panel glass, crystals form, resulting in an increased open porosity, density and inhomogeneous pore structure. The crystallization can, however, be inhibited by adding calcium phosphate, so enabling the preparation of high-quality foamed glass from flat glass or flat/container-glass mixture. The pore gas is predominantly CO₂ and the pressure inside the pores is 0.36–0.47?bar. The reduced effect of the composition on the foaming process suggests that there is a great potential for stabilizing the production of foamed glass and ensuring the product's quality.     

Prediction of breakthrough curves for adsorption on activated carbon fibers in a fixed bed

A new model is proposed to describe the removal of volatile organic compounds (VOC) from a gas stream passing through a bed packed with activated carbon fibers (ACFs). Toluene was used as the test compound. Both pore diffusion and surface diffusion are considered in the model. The equilibrium behavior is shown to fit the Dubinin–Radushkevich isotherm with the values of parameters K and W₀ of 1.101 × 10⁻⁹ and 57.73 kg/m³, respectively. The experimental results show that this model can predict VOC breakthrough curve very well.     

Magnetoelectronic properties of a graphite sheet

Magnetoelectronic structures of a two-dimensional (2D) graphite sheet are calculated by the tight-binding model. They are very sensitive to the magnitude of perpendicular magnetic field (B). B imposes the periodical boundary condition on the Bloch functions in the real and momentum spaces. Thus, B changes energy dispersions, energy spacing, bandwidth, and oscillation period of Landau levels. B could reduce the dimensionality of a graphite sheet. Energy dispersions mainly exhibit zero-dimensional (or 1D) characteristics. A lot of delta-function-like peaks (or square-root peaks) in the density of states can be clearly found. The magnetic field dramatically changes the joint density of states and the magnetoabsorption spectra. So, many peaks with different structures are produced.     

Influence of embedding Cu nano-particles into a Cu/SiO2/Pt structure on its resistive switching

Cu nano-particles (Cu-NPs) were embedded into the SiO₂ layer of a Cu/SiO₂/Pt structure to examine their influence on resistive switching characteristics. The device showed a reversible resistive switching behavior, which was due to the formation and rupture of a Cu-conducting filament with an electrochemical reaction. The Cu-NPs enhanced the local electric field within the SiO₂ layer, which caused a decrease in the forming voltage. During successive switching processes, the Cu-NP was partially dissolved, which changed its shape. Therefore, the switching voltages were not reduced. Moreover, the Cu-NPs caused a non-uniform Cu concentration within the SiO₂ layer; thus, the Cu-conducting filament should be formed in a high Cu concentration region, which improves switching dispersion. The Cu-NPs within the SiO₂ layer stabilize the resistive switching, resulting in a larger switching window and better endurance characteristics.     

On the great difficulty of intercalating lithium with a second element into graphite

Lithium is able to intercalate into graphite leading to various binary graphite intercalation compounds, that are well defined by their stage. Concerning the ternaries, there is little literature on the subject. Thermodynamical and structural data, that differ largely from those of the other alkali metals, lead one to foresee some serious difficulties in synthesising such ternary compounds. Many experiments have attempted to synthesise ternary graphite intercalation compounds with lithium, using successively very electronegative elements, then fairly electronegative species and lastly electropositive metals. Numerous results, that are wholly negative, are described in this paper. The calcium-lithium system only allows one to prepare a novel intercalation compound, that is a first stage ternary phase exhibiting a large interplanar distance. This latter suggests that the intercalated sheets consist of several superimposed atomic layers. The synthesis of this ternary is not easy, because it needs reagents of very high purity. It possesses the brightness of metals and its strong hardness is very unusual among graphite intercalation compounds. On the other hand, the charge transfer between the graphene planes and the intercalated sheets, that just allows the intercalation, is especially high, and much higher than the LiC₆ compound.     

Influence of electron concentration and temperature on fullerene formation in a carbon plasma

The influence of electron concentration in ionized carbon vapor on C₆₀ fullerene formation was theoretically investigated. By considering C₆₀ molecule assembly at one or two intermediate stages of cluster collision we determined the plasma parameters (temperature and electron concentration) optimal for the fullerene synthesis. The estimations made showed that waves of electron concentration in plasma changed the rate of C₆₀ fullerene assembling.     

Controlling the diameter distribution of carbon nanotubes grown from camphor on a zeolite support

Single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs, respectively) of controlled diameter distribution were selectively grown by thermal decomposition of a botanical hydrocarbon, camphor, on a high-silica zeolite support impregnated with Fe-Co catalyst. Effects of catalyst concentration, growth temperature and camphor vapor pressure were investigated in wide ranges, and diameter distribution statistics of as-grown nanotubes was analyzed. High yields of metal-free MWNTs of fairly uniform diameter (∼10 nm) were grown at 600-700 °C, whereas significant amounts (∼30%) of SWNTs were formed at 850-900 °C within a narrow diameter range of 0.86-1.23 nm. Transmission electron microscopy and micro-Raman spectroscopy reveal that camphor-grown nanotubes are highly graphitized as compared to those grown from conventional CNT precursors used in chemical vapor deposition.     

Production of reactive bio-silica from the combustion of rice husk in a tube-in-basket (TiB) burner

Rice husk, the main by-product of milling rice paddy and an agro-waste, is available in vast quantities in the rice-growing regions. On combustion, rice husk leaves behind about 20% ash composed essentially of bio-silica. This cheap and abundant source of reactive silica powder has numerous diverse applications, ranging from electronic-grade silicon to pozzolana cements. The phase composition of silica in the ash and its surface area depend critically on the temperature of combustion of the rice husk. A Tube-in-Basket (TiB) burner described here burns rice husk in a controlled manner to produce amorphous silica powder of high surface area, as characterized by X-ray and BET analyses.     

Sorption kinetics of eight gases on a carbon molecular sieve at elevated pressure

The sorption kinetics of eight different molecules (O₂, N₂, Ar, CO, CO₂, SO₂, CH₄ and H₂) on a carbon molecular sieve was studied over a wide range of pressures up to 15 atm by using a volumetric method. The acentric factor was suggested as a potential factor to estimate the relative sorption rate. Since the apparent time constants of all the components showed much stronger dependence of pressure than those expected by the traditional Darken relation and the structural diffusion model, new models with the diffusion relation at the supercritical condition was proposed to predict the kinetic behaviors in the micropores. The proposed model successfully predicted the apparent time constant up to high pressure. In addition, the semi-empirical model that combined acentric factor with the proposed model was able to predict the strong pressure dependence accurately. However, since the strong adsorbates, CO₂ and SO₂, showed two-stage kinetic behavior with pressure, which was different from that of the other adsorbates. The kinetic behaviors of these molecules could be predicted by using two different sorption models.